This invention relates to vacuum booster devices for boosting automotive brake master cylinder by vacuum pressure and, more particularly, to improvements in those of the type comprising a booster shell, a booster piston axially slidably accommodated in the booster shell, first and second working chambers defined in the booster shell separately from each other by the booster piston, the first working chamber being held in communication with a vacuum source, the second working chamber being selectively placed in communication with the first working chamber or the atmosphere through a control valve means, an input rod mounted on the booster piston for forward and rearward movement with respect to the latter and connected to the control valve means so as to produce a pressure difference between both the working chambers for causing the booster piston to follow the forward movement of the input rod, tie rod means extending through the booster piston for connection of front and rear walls of the booster shell, seal means arranged between the tie rod means and the booster piston allowing operation of the piston, and a return spring compressed in the first working chamber for reinforcing the retraction of the booster piston.
In the above-mentioned booster device, the booster shell can be protected from effect of the forward thrust loaded from output side by transmitting the thrust to the vehicle body through the tie rods so that such high rigidity as to bear the thrust loading need not be given to the booster shell, and advantageously, the shell can be formed light-weighted by using thin steel sheets, synthetic resins or the like as its base materials.
However, in this known device, if the weight of the booster shell is lessened to an excessive degree, the booster shell may be outwardly deformed by the resilient force of the return spring of the booster piston in case of an unsupported booster device, central portion of the front wall of the booster shell defining the first working chamber may be inwardly deformed under the sucking action of the vacuum pressure when the pressure is accumulated in the first working chamber.